Haiti Quake Offers Surprising Lesson About Earth's Surface

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The Hotel Montana, a four-star resort in Haiti, came crashing
down during the country's massive Jan. 12 earthquake — a
surprising collapse, since the hotel had been sturdily built on
presumably solid ground.

As Haiti continues to sift through the damage, scientists have
found that the earthquake's energy spread in an unusual way. It
turns out that the topography of the Earth's surface is just as
important as the ground underneath in determining how an
earthquake spreads, a study detailed in the Oct. 17 online
edition of the journal Nature Geoscience reveals. Two other
recent studies, also published in Nature Geoscience, found that
the Haiti quake was the result of a
rupture along a previously unknown fault and that it
generated several small tsunamis.

When the magnitude-7.0 earthquake rocked the country, more than
200,000 people died, 1.5 million were left homeless, and the
damage totaled between $9 billion and $14 billion. The
extent of the damage was not surprising, considering the
quake struck Port-au-Prince, which is filled with poorly
constructed buildings and sits atop a kind of rock that allows
the shaking to spread easily.

But one seemingly stable area, along a foothill ridge, suffered a
surprising amount of damage.

Ground motions during the Haiti earthquake were significantly
amplified along this mountain ridge, causing substantial
structural damage. Felled along with the Hotel Montana were two
United Nations buildings, the Hotel Christophe, the French
ambassador's home and a number of substantial private homes.

"It was a surprise," said Susan Hough, a study team member with
the U.S. Geological Survey in Pasadena, Calif. "It's a ridge in
an affluent part of the city, and it really got hammered."

Topography — the shape of the Earth's surface — is not generally
considered when geologists study an area's vulnerability to
earthquakes. Geologists typically focus on the kinds of rocks
underground.

In the new study, Hough and her colleagues measured aftershocks
following the Haiti quake to assess how ground motion varied in
different parts of Port-au-Prince.

Importance of topography

They compared sites located on soft sediments, hard ground and
rocky, mountainous ridges. As expected, structures built on soft
sediments experienced enhanced shaking. However, the strongest
ground motions occurred on the mountainous ridge where the Hotel
Montana collapsed.

"We suggest that microzonation maps can potentially be
significantly improved by incorporation of topographic effects,"
the authors write in their study. Microzonation maps subdivide
earthquake-prone areas into smaller zones of risk based on
particular factors, such as landslide susceptibility and ground
shaking.

Pinning down how topography amplifies
an earthquake's energy — be it by steepness or width of a
ridge, for example — will take longer, said the authors, but the
initial findings could help guide the rebuilding effort.

"When you rebuild, you're going to have to take these places into
account," Hough told OurAmazingPlanet.